Process for pickling of steel strip and regeneration of the contact acid



May 20, 1969 PROCESS FOR PICKLING OF STEEL STRIP AND REGENER M. W. ROBINSON, JR.` ETAL OF THE CONTACT ACID Filed Sept. 17. 1965 ATION Mew/LE W. ROBINSON JR., CLARENCE STEELMAN ./R. and ENocH PENN/Ns JR.

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orneys United States Patent O U.S. Cl. 134-3 3 Claims ABSTRACT OF THE DISCLOSURE Process for pickling steel wherein continuous lengths of metal are moved through a succession of interconnected compartments and then into a separate rinse area wherein hydrochloric acid pickling solution flows through the several compartments countercurrent to the travel of the metal. The spent solution from the last compartment in the series is withdrawn and processed to regenerate hydrochloric acid solution and recover ferric oxide, and the regenerated hydrochloric acid solution is returned to the rst compartment of the series at a constant strength. The solution in each compartment is separately heated by indirect heat, so that optimum temperatures may be maintained in each compartment.

The present invention is a continuation-in-part of our copending application Ser. No. 324,479, led Nov. 18, 1963, now Patent No. 3,310,435, dated Mar. 21, 1967.

The present invention relates to a process for pickling a steel strip by movement through a horizontally disposed pickling tank, wherein the hydrochloric acid-Water solution is moving in a direction countercurrent to the moving strip and increments of the used solution are continuously being withdrawn for treatment to remove the iron chlorides therein and return to the pickling tank.

The vapors attendant to operation of such a process are toxic and the pickling and rinse tanks should be suitably enclosed. Such enclosures are 'well known in the art and are not specifically disclosed herein.

The pickling solution is most effective when maintained at above atmospheric temperature, preferably between 140 F. and 200 F. Prior practice has been to inject live steam into the solution to maintain its temperature. One undesirable feature of this practice is the dilution of the solution by the condensate from the steam.

One object of the invention is to provide for uniform heating of the pickling solution without dilution of its acid content.

Another object of the invention is to provide for counterow of the pickling solution relative to the movement of the strip through the pickling tank.

A further object of the invention is to maintain substantially uniform acid strength of the solution Within the pickling tank by continuously withdrawing a portion of the used solution and replacing same with full strength solution.

A still further object of the invention is to rejuvenate the withdrawn solution by removal of iron chlorides therefrom and to return the recovered acid in the desired strength to the system.

These and other objects of the invention will be made apparent from the following description and the drawing forming a part thereof wherein the pickling tank, the apparatus associated therewith and the acid recovery apparatus are schematically shown.

3,445,284 Patented May 20, 1969 In the drawing the pickling tank is indicated generally at P, the rinse tank as R. Tank P is divided into a plurality of compartments 2, 3, 4 and S by vertically disposed walls 7, 8, 9 and 10. The tank P is initially suitably filled by feeding a substantially 20% hydrochloric acid and water solution from storage tank 38 through conduit 38a into tank compartment 2, from which it uniformly overows successively into compartments 3, 4 and 5 through suitable apertures A in walls 7, 8 and 9.

The strip S to be pickled is looped and drawn through the tank P by suitable pairs of rolls 6 mounted upon the vertical walls of the tank compartments and passed to a suitable rinse tank R from which it is passed, by rolls 6, to a suitable recoiling means for further processing.

The pickling solution in each tank compartment 2, 3, 4 and 5 is separately maintained at a suitable temperature, such as F. to 200 F. by withdrawing the solution from each tank compartment through conduits 15 by means of pump 16 and passing same through a suitable heat exchanger 14, 14a, 14b and 14c before returning the solution through conduit 16EL to the respective compartment. The heat exchangers 14 are of a suitable construction having an internal chamber through which the solution is passed and an enclosing area through which steam is passed by means of conduits 17 and 18. The live steam entering through conduit 17 and the steam and condensate being withdrawn through conduits 18 from which it is discharged for disposal. In this manner the solution being heated during passage through heat exchangers 14 is not diluted by such condensate.

A portion of the pickling solution within the tank P is continuously Withdrawn through compartment wall 10 by means of conduit 11 and pump 12, thereby inducing a tlow of solution from compartment 2 to compartment 5. The amount of withdrawn solution is returned to the tank compartment 2 by conduit 38a from acid storage tank 38, to maintain a substantially constant volume of solution in tank P during the pickling operation.

The pickling solution withdrawn from compartment 5 of tank P is discharged through pump 12 and conduit 11 into a suitable storage tank 13. From tank 13 the used solution is withdrawn through conduit 19 into pump 20 and discharged through conduit 21 into the top of roasting device 24 by means of conduits 22 and spray nozzles 23. Within roasting device 24 the discharged spray of used pickling solution is vaporized upon contact with the rising hot gases resulting from the combustion of fuel and air injected from inlet 25. As described in our copending application Ser. No. 324,479, above referred to, the reaction between the spray of solution and the hot combustion gases Within roaster 24 produces iron oxide solids and H2O and HC1 vapors. The iron oxide solids fall to the bottom of roaster 24 and are discharged through conduit 26 into storage receptacle 27. The water and HC1 vapors, plus any suspended iron oxide nes are withdrawn from roaster 24 through conduit 28 into a ccntrifuging device such as a cyclone 29 where the solid iines are removed and the water and HCl vapors pass off through conduit 30 into the base of an adiabatic absorber 31. Here the rising HC1 vapors are condensed in a downwardly owing stream of cool water and the water vapor is condensed or passes olf from the top of absorber 31. This function of the adiabatic absorber is well known and need not be further described other than to say that HC1 vapors and water combine to provide about a 20% acid content solution which is discharged through conduit 33 into storage tank 38.

For recovery of any solution carried by the strip S from the tank P to rinse tank R, water for use in the absorber 31 may be supplied by withdrawal of water from tank R through conduit 34, pump 35 and conduit 36 for discharge into the top of absorber 31 by conduit 3 32. Any additional water required is supplied from an suitable outside source through conduit 37, to maintain the regenerated solution at a constant concentration of about 20% acid.

Having thus described our process and schematically disclosed suitable forms of apparatus to practice same, it will be apparent to those skilled in the art that ferrous strip to be pickled may contain oxide surface coatings of different degrees of thickness, hardness, etc. Thus required solution temperature and rate of solution withdrawal may vary under changing conditions.

We claim:

1. A continuous pickling process for pickling continuous lengths of rolled metal with hydrochloric acid solution and regenerating fresh hydrochloric acid pickling solution from the spent pickle liquor and reusing it cornprising:

(a) continuously circulating hydrochloric acid solution through a horizontal succession of pickling compartments from the forward end of the series to the last of the series,

(b) moving the continuous length of metal in succession through the series of compartments counter to the direction of ow of the hydrochloric acid picklng solution and then through a water rinse area,

(c) separately heating the solution in each compartment in a controlled range between about 140 F. and 200 F. by indirect heating,

(d) continuously withdrawing the pickle liquor from the last compartment of the series at a uniform rate and regenerating therefrom a 20% hydrochloric acid solution and recovering therefrom ferrie oxide, and

(e) returning the regenerated acid solution to the lirst compartment ofthe series.

2. A continuous pickling process as defined in claim 1 in which the spent pickle liquor is contacted in finely divided form with hot combustion gases and the resulting iron oxide is separated from the combustion gases and the gases passed into an absorption column in which they encounter a controlled counter ow of water.

3. The method defined in claim 2 in which water flow to the absorber and water in the spent pickle liquor are maintained at a level where the resulting acid solution is substantially constant at about 20% concentration.

References Cited UNITED STATES PATENTS 3,129,712 4/ 1964 Thomas 134-64 XR 1,544,506 6/1925 Tytus 134-28 XR 2,428,221 9/ 1947 Hudson 134-3 2,668,130 2/1954 Martin 134-3 XR 2,709,143 5/1955 Francis et al 134-15 XR 3,100,727 8/1963 Labergere 134-28 XR 3,310,435 3/ 1967 Robinson et al. 134-3 FOREIGN PATENTS 838,261 6/ 1960 Great Britain.

MORRIS O. WOLK, Primary Examiner.

JOSEPH T. ZATARGA, Assistant Examiner.

U.S. Cl. X.R. 134-10, 15, 28, 41 

